The present invention relates to a signal transmission system and a receiving apparatus each using an orthogonal frequency division multiplexing (hereinafter abbreviated as OFDM) scheme for transmitting information codes by means of a plurality of carriers orthogonal to one another and more particularly to a receiving apparatus for the signal transmission apparatus of the OFDM scheme in which the plurality of carries of the OFDM scheme are modulated in accordance with a modulation scheme accommodated to the synchronization detection (hereinafter referred to as synchronous modulation scheme).
In recent years, in the field of radio devices, the OFDM scheme is in the limelight as a modulation scheme immune to multipath fading. A large number of applied studies on the OFDM are now in progress in the fields of next-generation television broadcasting, FPU (Field Pick-up Unit), radio LAN and so on in many countries including European countries and Japan. The trend of development and the system for terrestrial-wave digital broadcasting of the UHF band are disclosed in “THE JOURNAL OF THE INSTITUTE OF IMAGE INFORMATION AND TELEVISION ENGINEERS”, 1998, Vol. 52, No. 11, pp. 1539-1545 and pp. 1658-1665.
As an example of the prior art, the UHF-band terrestrial-wave digital broadcasting system in Japan will be described below. It should be noted however that this system involves an extremely complicated configuration, so that the following description will be made on the system which is simplified to such an extent that is required for understanding the present invention.
Beginning with description on the structure of a carrier in this broadcasting system, as illustrated in FIG. 7, one signal transmission frequency band is divided into 13 segments, and a total of approximately 1,400 carriers are used in the transmission.
Information codes of up to three channels (three layers) can be simultaneously transmitted as a signal to be transmitted. The number of segments and a modulation method used in each layer can be freely selected as mentioned in the above document.
As modulation schemes, there are two kinds of modulation schemes using synchronous detection (synchronous modulation scheme) and using differential detection (differential modulation scheme).
Since the present invention concerns the transmission apparatus using the synchronous modulation scheme, a 64 quadrature amplitude modulation (64QAM) scheme, for example, is used as the synchronous modulation scheme and a structure of a carrier portion for modulating all segments in accordance with the 64QAM scheme will be described in detail.
FIG. 8 is a diagram for explaining in greater detail the structure of the carriers of segments which are modulated in accordance with the synchronous modulation scheme. The diagram of FIG. 8 shows only left side (lower frequency region) of the band W shown in FIG. 7.
In a mode which uses all segments for transmission of information codes on one layer, it may be through that a similar structure is repeated over the entire band.
In FIG. 8, the horizontal direction represents the frequency and the vertical direction represents the lapse of time, and squares “□” arranged in the horizontal and vertical directions each represent one carrier. Thus, one row of squares “□” arranged in the horizontal direction within the band W represents one symbol which forms an OFDM signal.
The square “□” with “SP” inscribed therein represents the carrier position of a pilot signal which is used for reproducing a reference signal (utilized as the basis for phase and amplitude) during demodulation. Further, the square “□” without any inscription therein represents the carrier position of an information code modulated in accordance with the 64QAM scheme.
The pilot signals are scattered both in the frequency direction and the time direction and accordingly they are designated as SP (Scattered Pilot).
FIG. 8 merely schematically illustrates the arrangement of the SP, and a TMCC (Transmission and Multiplexing Configuration Control) carrier for transmission of a control signal and additional information AC (Auxiliary Channel) to be arranged originally in FIG. 8 are omitted.
Further, in the terrestrial-wave digital broadcasting system, the interval in the horizontal direction of the carriers having the SP in the time direction is three, whereas the interval is changed to five in FIG. 8. This change is made for easy understanding of the present invention described later and essential contents are identical.
A signal modulated with the 64QAM scheme is represented by any position of 64 signal points indicated by broken line circles on a complex plane defined with I-axis (real axis) and Q-axis (imaginary axis) orthogonal with each other as shown in FIG. 9 and the respective signal points are corresponded to 6-bit codes which are different from one another. For example, the signal point b on the I-Q complex plane in FIG. 9 is corresponded to a code “011111”.
The modulation processing in accordance with the 64QAM scheme involves dividing a sequence of input information codes in units of six bits, each divided 6-bit code is assigned to one of the 64 signal points on the I-Q complex plane, for example, a signal point indicated by a solid line circle “◯”. Each of 6-bit codes is converted into a signal corresponding to coordinate value of the assigned signal point.
On the other hand, a received modulated signal is affected by noise and other influences during a transmission process and distorted. For example, a signal point upon transmission indicated by a solid line circle “◯” in FIG. 9 is moved from a position b to a position b′ indicated by a cross “×” when received.
The demodulation processing in accordance with the 64QAM scheme involves selecting the signal point b closest to the signal point b′ for the received signal indicated by cross “×”, from 64QAM signal points indicated by broken line circles in FIG. 9, and outputting a 6-bit code corresponding to the selected signal point.
For the demodulation processing, the correct signal point position indicated by the broken line circle associated with the received signal must be detected. The reproduction of the position only requires to find, for example, the direction and magnitude of a reference signal vector representing a correct position of a reference coordinate point a on the signal space in FIG. 9.
The direction and magnitude of the reference signal vector of a received signal are affected by multipath and the like, which may occur on a transmission path, causing the phase to rotate and the amplitude to change as well, as shown in FIG. 10. It is necessary to reproduce the reference signal vector on basis of the pilot signal for each carrier on the reception side. The reference signal vector is produced for a carrier containing no pilot signal on the basis of the pilot signal contained in a nearby carrier.
As described above, the phase and magnitude of the reference signal vector change every time or from one carrier to another, while the manner of changing is generally expressed by a smooth curve and has a strong correlation in the time direction and in the carrier direction. Accordingly, the reference signal vector for a modulated signal A of an arbitrary carrier of an arbitrary symbol in FIG. 8 can be calculated by interpolation of a plurality of sporadically transmitted SP signals. In FIG. 8, the SP signals are arranged so that the interpolation can be made efficiently.
The terrestrial-wave digital broadcasting system has no regulation to the method of reproducing the reference signal vector from a signal having the carrier structure of FIG. 8. However, it can be realized by the circuit shown in FIG. 11, for example. FIG. 11 shows a circuit portion used to reproduce the reference signal vector in a receiving apparatus of the OFDM scheme.
A received signal produced from a fast Fourier transform (FFT) circuit 5 is supplied to a time-direction interpolation circuit 6 and a delay circuit 7. The time-direction interpolation circuit 6 extracts the pilot signals SP from the received signal. The pilot signals SP are subjected to filtering processing in a digital LPF having a predetermined number of taps for each carrier row containing the pilot signals in the time direction as shown by hatching of FIG. 12 and outputted as reference signal vector signals interpolated in the time direction. The digital LPF is not shown in FIG. 11, while it is included in the time-direction interpolation circuit 6. Each tap coefficient of the digital LPF is stored in a coefficient memory (not shown) in the time-direction interpolation circuit 6. The carrier in which the SP signal is arranged is hereinafter referred to SP carrier.
FIG. 13 schematically illustrates the interpolation method of determining reference signal vectors of carriers having no SP by means of the interpolation in the time direction for carriers existing on a single-dotted broken line 3 of FIG. 8. The horizontal axis represents time and the scale on the horizontal axis is represented for each symbol. The vertical lines having mark ◯ at the top thereof represent signal vectors of the received SP signals.
The reference signal vectors of carriers received from the time that a certain SP, for example, SP1 is received to the time that next SP2 is received are calculated by interpolation by means of an LPF having a fixed number of taps by using signal vectors of a plurality of SPs positioned before and after the carriers.
All of reference signal vectors of the carrier row in the time direction hatched in FIG. 12 can be calculated by the interpolation of the SPs in the time direction. At this time, the interpolation in the LPF requires signals having the symbols equal in number to the taps and the interpolated signals are delayed by the number of symbols equal to about half of the number of taps to be outputted.
A delay circuit 7 is provided to match the timing of the received signal to the timing of the interpolated signals.
On the other hand, the reference signal vector for a modulated signal A in the carrier in which SP is not arranged in FIG. 8 is calculated by interpolation in the frequency direction of the reference signal vectors of SPs arranged in the time axis direction and SPs calculated by interpolation of thereof. A frequency-direction interpolation circuit 8 of FIG. 11 is a circuit for carrying out the interpolation.
FIG. 14 schematically illustrates the interpolation method of the SPs in the frequency direction for the symbol on a single-dotted broken line 4 of FIG. 12. The horizontal axis represents frequency and the scale on the horizontal axis is represented for each carrier position. Thick arrows represent the reference signal vectors W(1), W(5+1), W(2×5+1), . . . for the carriers hatched in FIG. 12 and calculated by interpolation in the time axis direction. The numerals within parentheses are carrier numbers.
The reference signal vectors at the carrier positions A having no thick arrow are calculated as follows. First of all, signals W(1), 0, . . . , 0, W(5+1), 0, . . . , 0, W(2×5+1), . . . obtained by setting the magnitudes of vectors of the carriers having no thick arrow in FIG. 14 to 0 are caused to pass through a usual digital LPF (not shown) having 23 taps, for example, so that the smooth interpolated signals expressed by a broken line are calculated. The interpolated signals thus calculated are outputted as the reference signal vectors of the modulated signal A. The digital LPF is provided within the interpolation circuit 8.
The reference signal vector signals reproduced by the frequency-direction interpolation circuit 8 and the received signal delayed by the number of symbols equal to about half of the number of taps in the delay circuit 7 are supplied to the 64QAM demodulation circuit 9 to correct the phase and amplitude of each signal point of the received signal, so that the signal point position deformed as shown in FIG. 10 can be corrected to the correct position as shown in FIG. 9 to thereby demodulate information codes.